This invention relates to papermakers' fabrics and especially to papermaking fabrics for the forming section of a papermaking machine.
In the conventional papermaking process, a water slurry or suspension of cellulose fibers, known as the paper "stock", is fed onto the top of the upper run of a traveling endless forming belt. The forming belt provides a papermaking surface and operates as a filter to separate the cellulosic fibers from the aqueous medium to form a wet paper web. In forming the paper web, the forming belt serves as a filter element to separate the aqueous medium from the cellulosic fibers by providing for the drainage of the aqueous medium through its mesh openings, also known as drainage holes, by vacuum means or the like located on the drainage side of the fabric.
After leaving the forming medium, the somewhat self-supporting paper web is transferred to the press section of the machine and onto a press felt, where still more of its water content is removed by passing it through a series of pressure nips formed by cooperating press rolls, these press rolls serving to compact the web as well.
Subsequently, the paper web is transferred to a dryer section where it is passed about and held in heat transfer relation with a series of heated, generally cylindrical rolls to remove still further amounts of water therefrom.
Over the years, papermakers have sought improvements in the forming fabric, not only with respect to the operating life of the fabric, but also with respect to the quality of the paper sheet produced on it. Triple layer fabrics were introduced for this purpose. The triple layer fabric has two generally distinct surfaces. The top surface is one integral fabric structure designed specifically for papermaking to achieve the best possible sheet quality and machine efficiency. This top fabric is manufactured as an integral part of a woven structure with a completely separate bottom fabric designed specifically for mechanical stability and fabric life. The purpose of triple layer fabric development is to eliminate the compromises which exist with both single and double layer forming fabrics so that papermakers can produce the best possible paper sheet for top quality at reduced cost without sacrificing the wear characteristics of the papermaking fabric.
The paper produced on the papermaking machine is described in part with relation to its formation and wire mark. Formation is most commonly described as the difference in density of a sheet of paper when looking through the sheet. The ideal formation is a sheet which has completely uniform density. Sheets with areas of varying density are said to be flocky or cloudy. The word formation is generally used to describe macro scale areas of varying density which can be easily seen by the human eye. Headbox design and performance have the most effect on large scale formation. This, together with the turbulence created by stationary elements, principally dictates the final large scale sheet formation. Wire mark, on the other hand, is used to explain the micro or finer levels of density difference, often caused by the structure of the forming fabric on which the sheet was produced.
The initial fiber mat formed on a papermaking fabric, which becomes the paper sheet, is very greatly influenced by the surface structure of the filtering medium on which it settles. It follows that a fine, uniform support grid will give a more uniform initial fiber mat than a coarse non-uniform support grid. This degree of uniformity in fact influences subsequent layers of fiber as the sheet is formed, and eventually, the paper sheet produced.
The papermaking fabric is essentially a filter by which the cellulose fibers, of varying lengths, are separated from the water component of the paper stock. A completely closed fabric, or 100 percent closed fabric, would have no drainage and would therefore be unworkable. The fabric must be opened from this maximum, to create an orifice effect to allow drainage. A forming fabric which is 100% open is also no good as it will not retain fibers from the stock solution to form a sheet. Opening the fabric, additionally, often accomplished by reducing the diameter of the yarns used to weave the fabric, creates density differences.
The effect of differences in density of the paper sheet, whether caused by large scale flock or finer scale wire mark, is to vary the degree to which ink penetrates the paper sheet. FIG. 1 illustrates the way in which this phenomenon is caused. FIG. 1A illustrates that when a sheet is being formed on an open forming medium, the sheet will be made up of thick areas over the holes and thin areas over the knuckles. In FIG. 1B, during pressing and calendering, the thick areas are compressed more than the thin areas, which results in a sheet having differences in density. The paper of the resulting sheet, as shown in FIG. 1C, will have a high gloss, be very smooth and have low porosity in the areas of high density. These areas, when printed, will have low ink penetration which will result in a print in this areas which will have high gloss and possibly high offset. On the other hand, the areas of the sheet over the knuckles will have low density, low gloss, be rougher and have higher porosity. When printed, these areas will have greater ink penetration, which will result in a matt finish compared to the dense areas over the holes of the fabric, and with the high porosity, print strike through may occur to the opposite side of the sheet. Whether differences in density of the sheet are caused by large scale flock or fine scale wire mark, the effect on the final print quality of high and low gloss through variation in ink penetration is the same. Terms used to describe these effects are "galvanizing" or "mottle".
The type and pattern of wire mark that will be produced by any fabric can be easily shown by taking a surface impression of the papermaking surface of the fabric It has been found that the high knuckles of a fabric, around which the stock slurry flows and settles lower down in the fabric body, leave light areas. The degree of wire mark that hits the eye, therefore, is determined by the frequency and continuity of the pattern formed by the knuckles of the fabric. Openness of the fabric will, of course, affect these density variations and the surface impression.
For example, a coarse single layer fabric has low frequency, and each hole formed by the knuckle will therefore show up more than when compared to the higher frequency of the finer mesh. Further, if the wire mark pattern is a straight twill line, as compared to a broken satin, it will strike the eye to an even greater extent. The degree of differences in density of a sheet caused by wire mark, therefore, can be said to be affected by the frequency, or number of knuckles/square inch, and the continuity and coarseness of the pattern.
At the present time, there is a great need for a paper sheet with more uniform formation, and equal printing properties on both sides for every printing grade. It has been found that the micro density differences of the paper sheet, resulting from the knuckles of the yarns on the forming fabric, are the main cause of the problem. The perfect print is one where all the ink applied absorbs into the sheet at the same rate. To date, surfaces are far from uniform, as explained above, thus leading to differences in contact and absorption of ink depending on whether it lands on a light area over a knuckle or a heavy area over a hole. When the ink hits a particular area over a knuckle, it penetrates the sheet very easily, and if the volume is sufficient, will strike through to the other side. To achieve the best print, the printer has to modify his printing conditions to strike a balance between the two extremes.
It has been found that the key to the reduction, or elimination, of these printing problems can be achieved by careful selection of the papermaking fabric upon which a paper sheet is to be produced.
It is therefore an object of the present invention to prepare a papermaking fabric that produces a paper sheet of superior print quality.
Another object of the present invention is to provide a papermaking fabric that combines good drainage capability with an optimal paper sheet surface.
It is another object of the present invention to provide a papermaking fabric in which density differences are minimized in order to optimize the printing properties of the paper sheet formed thereon.
A further object of the present invention is to provide a papermaking fabric with good wear life and abrasion resistance that produces a paper sheet with optimal printing properties.
A further object of the present invention is to provide a method for making a paper sheet having minimal density differences.
It is a further object of the present invention to provide a papermaking fabric which relates its drainage orifice dimensions to the average length of the fibers to be used to form the sheet of paper.
Still another object of the present invention is to relate drainage orifice dimensions to average fiber length in order to control the degree of retention of fibers.